Valves, such as gate valves, when subjected to extremes of service, such as high pressures (e.g. 30,000 psi), elevated temperature (e.g., 300.degree. F.), cycling of the pressure and temperature, degrading fluids, and either reciprocation, rotation or both of the stem, experience frequent failure of the stem seal.
One effort to solve this problem is disclosed in U.S. Pat. No. 4,262,690 which suggests a packing assembly having a stack of packing rings formed of a material such as polytetrafluoroethylene with packing retainer rings on each side of each packing ring and metal chamfer rings on opposite sides of the assembly cammed against the stem and bonnet to prevent extrusion of the packing along the stem and bonnet.
Tapered expansion rings are used in U.S. Pat. No. 1,826,967 to maintain uniform packing loading when subjected to temperature changes. U.S. Pat. No. 4,288,082 uses a tapered surface to cam a metal seal ring into sealing engagement in the casing annulus of a well bore. U.S. Pat. No. 4,185,840 discloses a shaft seal including a stack of alternate elastomeric and rigid rings banded together with the elastomeric rings being progressively thinner in a direction away from the high pressure side of the seal to provide a uniform distribution or radial packing pressure along the length of the seal assembly.
U.S. Pat. No. 4,082,300 discloses a sealing assembly for a pump plunger for service in pumping corrosive liquids. The sealing assembly includes many types of metal, hard burned coal and flexible graphite rings.